If I maybe of assistance, beside being a timber wright, one of my other passions is stone, and all it's many enjoyments. I just love the stuff, from climbing it to carving, if we get a good job, I enjoy carving and dressing the stone plinths on a job, as much as the timber work, and rarely I get to carve joints in stone, like scarfs, which is just so fun!!!
I have dress a number of grist stone in my day and even helped carve a replacement for one. So much fun. I helped make a sharpening stone, (Arkansas hard sand stone) many years ago. It sat in a wood frame with all wood mechanicals and foot treadle for power. It was the beginning of another passion.
Now "dressing" and "balancing" the stone is critical for good operation, as you well know. Doing this by hand is arduous work, to say the least, especially without glazing or "burning" the stone. Spinning the stone while being kept moist and using a harder stone to ware down the high spots. Uhg! That is grunt work on a fresh stone.
Now, if you are not a purest, and want to get the job done in minutes, (if not seconds,) there is a stone trade trick to doing it. It is a little intimidating, but is not as bad as it sounds. You will need a 4" electric angle grinder and a diamond grinding disk used in stone carving. In a pinch, you can use a carborundum grinding stone wheel made for the electric angle grinder. Please note these can fragment and break if not of better quality and also burn the grind stone you are dressing. Below are some of my favorite suppliers, in order of favorites, should you want to pursue the craft further.